Embedded steel in reinforced concrete is normally protected against corrosion by virtue of a dense oxide film which forms on the steel surface in alkaline environments. This film acts as a barrier to aggressive agents. However, when concrete becomes contaminated with chloride ions, or when its alkalinity is reduced by absorption of carbon dioxide from the air, the passivating oxide film may break down, thus rendering the embedded steel subject to corrosion.
Much research has been done to examine the causes and mechanisms involved in the corrosion of steel reinforcement in concrete. The general consensus today is briefly that the corrosion process is electro-chemical in nature, in that sites where the passive oxide film is broken form anodes, and the surrounding areas where the film is still intact form cathodes. The anodic and cathodic areas together form corrosion cells leading to the dissolution of iron at the anodic areas.
Various electro-chemical methods have been developed in an effort to control this corrosion, or to neutralize its causes. One well known such method is that of cathodic protection whereby the embedded steel is brought to and maintained at an electrical potential at which it cannot corrode. Cathodic protection installations have been shown to be workable, but suffer from a number of adverse factors, not the least of which is their necessarily being permanent installations requiring ongoing monitoring and maintenance. Other disadvantages are high cost, the extra structural loading introduced by heavy concrete overlays, and the difficulty of ensuring correct current distribution.
Another such method is that of chloride extraction, in which chloride ions are caused to migrate under the influence of an electric field to an external electrolyte where they accumulate in, and eventually are removed with, the electrolyte. The Vennesland et al. U.S. Pat. No. 4,032,803 is an example of such processes. The chloride extraction process, though effective and less costly than cathodic protection, and thus a substantial improvement thereover, nevertheless suffers from the difficulty of predicting the time necessary for treatment to be completed. Because of this, frequent sampling and analysis of the concrete is required to determine remaining chloride levels. This difficulty is compounded by there so far being no residual chloride level which is generally accepted by the industry as being safe with regard to future chloride attack. These factors can make it difficult to calculate the cost and time necessary to reach a particular treatment target. In some cases, this time can also be unacceptably long from a practical aspect, especially since it is difficult to plan for in advance.
A third such method, which is applied to carbonated concretes, is the impregnation of the carbonated zones by the electro-migration of alkaline substances from an external source. The Miller et al. U.S. Pat. No. 4,865,702 is illustrative of this process. This latter method, though successful in carbonated concretes which are low in chloride, can become inefficient, or even fail, when the concrete contains significant amounts of ionic substances such as chlorides. Also, when the concrete contains blast furnace cement, or where pozzolans have been added to the mix, the treatment time can become unreasonably long. This is also the case when chloride accelerators have been used in the concrete mix and chlorides consequently are distributed throughout the entire concrete mass.
In practice it has been observed that it is both difficult and uneconomic, in many treatment situations, to reduce the chloride content of concrete to below about 50% of the original content. The documenting, monitoring and controlling of the chloride removal process involves the taking of numerous core samples from the concrete mass and analyzing the cores for chloride content. Concrete is a notoriously inhomogeneous material, so that statistically significant numbers of core samples need to be taken and analyzed to ensure effective monitoring of the removal process. Then, of course, the taking of each core sample leaves a hole to be filled. Similar considerations apply to the realkalization of concrete, in that drilled core samples are required for phenolphthalein testing and sodium and potassium determination.
The present invention overcomes the difficulties of the above mentioned methods by being highly predictable with regard to treatment time, by eliminating the necessity for sampling and chloride analysis, by being quicker and hence more economical to apply, and by being equally applicable to almost any kind of concrete, carbonated or not, chloride contaminated or not, pozzolanic or not, and whether or not blast furnace cement has been used.